strings: document that order of pairs matters in NewReplacer

Update NewReplacer documentation to specify that in the case of
multiple matches at the same position, the matching old/new
pair that appears first in NewReplacer arguments takes precedence.

Fixes #32699

Change-Id: I9d0616d28e5cd8c9bfa301be201f2b0ebf361dff
Reviewed-on: https://go-review.googlesource.com/c/go/+/185099


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>